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From: "Fred. Zwarts" <F.Zwarts@HetNet.nl>
Newsgroups: comp.theory,comp.lang.c
Subject: Re: Every D(D) simulated by H presents non-halting behavior to H ###
Date: Sat, 18 May 2024 18:01:39 +0200
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Op 18.mei.2024 om 17:44 schreef Richard Damon:
> On 5/18/24 11:34 AM, James Kuyper wrote:
>> On 5/18/24 09:02, Mikko wrote:
>>> On 2024-05-17 17:14:01 +0000, olcott said:
>>
>> I recommend ignoring olcott - nothing good ever comes from paying
>> attention to him.
>>
>>>> On 5/17/2024 5:53 AM, Mikko wrote:
>>>>> On 2024-05-16 14:50:19 +0000, olcott said:
>>>>>
>>>>>> On 5/16/2024 5:48 AM, Mikko wrote:
>>>>>>> On 2024-05-15 15:24:57 +0000, olcott said:
>> ...
>>>>>>>> typedef int (*ptr)();  // ptr is pointer to int function
>>>>>>>> 00 int H(ptr x, ptr x);
>>>>>>>> 01 int D(ptr x)
>>>>>>>> 02 {
>>>>>>>> 03   int Halt_Status = H(x, x);
>>>>>>>> 04   if (Halt_Status)
>>>>>>>> 05     HERE: goto HERE;
>>>>>>>> 06   return Halt_Status;
>>>>>>>> 07 }
>>>>>>>> 08
>>>>>>>> 09 int main()
>>>>>>>> 10 {
>>>>>>>> 11   H(D,D);
>>>>>>>> 12   return 0;
>>>>>>>> 13 }
>>>>>>>
>>>>>>> Can you find any compiler that is liberal enough to accept that?
>>>>>>>
>>>>>>
>>>>>> It has been fully operational code under Windows and
>>>>>> Linux for two years.
>>>>>
>>>>> If your compiler does not reject that program it is not a conforming
>>>>> C compiler. The semantics according to C standard is that a diagnostic
>>>>> message must be given. The standard does not specify what happens if
>>>>> you execute that program anyway.
>>>>>
>>>>
>>>> It is not nit picky syntax that is the issue here.
>>>> The SEMANTICS OF THE C PROGRAMMING LANGUAGE SPECIFIES
>>>>
>>>> No D simulated correctly by any H of every H/D pair specified
>>>> by the above template ever reaches its own line 06 and halts.
>>>
>>> The standard allows that an program is executed but does not
>>> specify what happens when an invalid program is executed.
>>
>> You've cross-posted this to comp.lang.c after a long-running discussion
>> solely on comp.theory. Presumably you're doing that because you want
>> some discussion about what the standard says about this code. For the
>> sake of those of us who have not been following that discussion on
>> comp.theory, could you please identify what it is that you think renders
>> this code invalid? Offhand, I don't see anything wrong with it, but I'm
>> far more reliable when I say "I see an error" than when I say "I don't
>> see an error".
>>
>>
>>>> Fully operational software that runs under Widows and Linux
>>>> proves that the above is true EMPIRICALLY.
>>>
>>> No, it does not. As the program is not strictly comforming
>>> and uses a non-standard extension some implementation may
>>> execute it differently or refuse to execute.
>>
>> Which non-standard extension does it use?
> 
> I think the issue is the casting of a pointer to function to a pointer 
> to object, which is one of the grey areas in the standard. (which occurs 
> in code not shown)
> 
> It is not specified that such a cast is allowed, but it also isn't 
> specifically disallowed, it is just omited as a case in the listing of 
> te possibilities for casting.
> 
> POSIX requires that certain limited object pointers can be cast to a 
> function pointer, but that is an extension.
> 
> Most common architectures will support it as they are both just "memory 
> addresses" into the same memory space, but that is not promised by the 
> standard.

Another issue seems to be that in the declaration of H:
int H(ptr x, ptr x);
both parameters have the same name.
(Olcott is famous for using the same name for different objects.)